Plastic deformation behavior of pre-twinned CoCrFeNi high entropy alloy under quasi-static and dynamic deformation

Multiple deformation mechanisms in face centered cubic high entropy alloys (HEAs) have been demonstrated to be induced easily by either extreme condition deformation or composition tuning. It seems to be a challenge to induce multiple deformation behaviors by microstructure design, and the underlying mechanisms are still unclear. Here, we architected a pre-twinned structure in the CoCrFeNi HEA via powder metallurgy in combination with cryogenic deformation and annealing processes. The deformation behaviors and dislocation structure evolution under quasi-static tension and dynamic compression were investigated. It was found that deformation twins and microbands were triggered during quasi-static tensile deformation due to the enhanced flow stress by the strengthening contributions of high density of pre-twins and in-situ nanoparticles, resulting in a simultaneous improvement of strength and ductility. Under dynamic compression, the higher flow stress promoted deformation twinning occurred in both pre-twinned and non-twinned samples, leading to a high true stress of 1.46 GPa with a good true strain of 70.5%. Such superior strain ability prolonged the evolution of dislocation structures, thereby leading to the transition from microbands to dislocation cells.